The present invention relates to an Al alloy valve spring retainer which is used in a valve operating mechanism of an internal combustion engine.
FIG. 2 illustrates one example of a valve operating mechanism of an internal combustion engine, in which to the upper end of a poppet valve 1 a valve spring retainer 3 is attached via a pair of semi-cylindrical cotters 2. Between a cylinder head (not shown) and the lower surface of a spring contacting flange 3a of the valve spring retainer 3, a valve spring 4 is provided, and the poppet valve 1 is usually energized upwards via the valve spring retainer 3. The numeral 5 denotes a rocker arm which contacts the upper end of the poppet valve 1 and which is shaken vertically by a rotary cam (not shown), thereby driving the valve in a vertical direction.
The valve spring retainer 3 used in such a valve operating mechanism is made of Al alloy to decrease inertia mass of the valve operating mechanism instead of steel which is usually used.
The Al alloy valve spring retainer is usually made by cold forging such as T6 treatment under JIS (Japanese Industrial Standard) and machining. T6 treatment under JIS means the steps of heating at about 500.degree. C. for several hours, cooling rapidly by water quenching and heating for several hours between 100 and 200.degree. C.
The valve spring retainer 3 is subjected to large repeating loads by the valve spring 4. Therefore, the flange 3a which is engaged with the valve spring 4 requires high fatigue strength and wear resistance. Such Al alloy spring retainer is softer than steel one, and mechanical strength is improved by T6 treatment to increase fatigue strength and wear resistance. However, machining such as lathes is made as finish on the whole inner and outer circumferential surface, so that fatigue strength and wear resistance are decreased.
This is because machining made after T6 treatment increases surface roughness and involving notch effect is likely to cause stress concentration to decrease fatigue strength. It is found that decrease in wear resistance is because a hardened surface layer formed by T6 treatment such as oxidizing coating layer is cut off and lost by machining.